background-gradual(direction, color1, color2)
  background: -webkit-linear-gradient(direction, color1, color2) /* Safari 5.1 - 6.0 */
  background: -o-linear-gradient(direction, color1, color2) /* Opera 11.1 - 12.0 */
  background: -moz-linear-gradient(direction, color1, color2) /* Firefox 3.6 - 15 */
  background: linear-gradient(direction, color1, color2) /* 标准的语法 */
  background: -webkit-gradient(linear, direction top, right top, from(color1), to(color2))
